The present invention concerns a filter-replacement cassette for an extruder. The cassette has a housing. The housing has at least one bore. The bores extend above and below and at a right angle to the horizontally flowing plastic. A cartridge that accommodates a replaceable filter slides back and forth inside each bore.
Filter-replacement cassettes are positioned downstream of the outlet from an extruder, and the thermoplastic melted in the extruder initially travels through a simple or composite filter that filters out contamination. The filter or filters, and the perforated disk they rest against, are accommodated in a replaceable cartridge. One cassette which contains only a single filter cartridge is sometimes employed, so that the extrusion process has to be interrupted to change the filter. Filter-replacement cassettes with one cartridge above another are also known, with the extrudate traveling through both. One of the two cartridges is always blocked off during the replacement process, and the extrusion process is not interrupted.
There is a drawback, however, in that, when a new section is to be extruded or especially when a different plastic is to be employed, a considerable quantity of defective plastic section will initially be extruded that should not be allowed into the gauging tool downstream of the extrusion head. Start-up valves and outlets are accordingly employed in the vicinity of the extrusion head, and the waste is diverted through them.
Another drawback to the known approach to replacing filters is that the filter-replacement cassette must be separate from the diversion mechanisms, which makes the cassettes relatively expensive.
The principal object of the present invention is to employ existing filter-replacement cassettes to simplify the start-up procedure in an extruder.
This object is attained in a filter-replacement cassette of the aforesaid type wherein the cartridge has at least one tap permanently integrated into it in addition to the replaceable filter, the housing accommodates either a cartridge or a start-up outlet bore and, when it accommodates the cartridge and while the cartridge is in a prescribed position, the start-up outlet bore can communicate with the cartridge""s tap and with an intake channel for the extrudate.
The concept behind the present invention is to provide the cartridge, which can be longer than a conventional cartridge if necessary, with a means of diversion, specifically the tap, to thereby allow it to assume, in addition to the traditional operating and filter-replacement positions, another and specifically defined position wherein the tap intercepts the waste extruded during start-up.
The present invention allows both discontinuous operation (with a single cartridge) and continuous operation (with two cartridges). In the latter event, it preferably is the lower cartridge that has the tap.
The present invention also features an embodiment with a single start-up outlet bore. The tap in this embodiment provides communication between an outlet bore at the end of the cartridge and the intake line.